Sound recording and reproducing system



Oct. 31, 1944. Q s TT 2,361,658

SOUND RECORDING AND REPRODUCING SYSTEM I Filed Oct. 26, 1942 T'li il.

OSCILLATOR T0 RECORDER AMPLIFIER TO SOURCE of AUDIO WAVES AMPL.

INVENTOR CHESTER M.

DISCRIMINATOR RECTIFIER R E W L P M A AMPLIFIER T0 .70 RECORD PICKUP.SINNETT vww A'TroRNEY Patented Oct. 31, 1944 SOUND RECDRDING AND SYSTEMnnrnonucmc .Chester M. Sinnett, west'montn. 1., asslgnor to RadioCorporation of America, a corporation of Delaware Application October26, 1942, Serial No. 463,349

6 Claims.

My present invention relates to the recording of frequency modulatedsignals,- and more particularly to a novel and improved type of systemfor recording and/or reproducing frequency modulated oscillations.

It has been proposed in the past by W. V. B. Roberts, in applicationSerial Number369,829, filed December 12, 1940, to record sound waves byfrequency modulating carrier energy with such sound waves, and theresulting frequency modulation signal energy then being recorded. Suchrecords are reproduced in a system employing a discriminator-rectifiernetwork commonly employed in the art of receiving radiated frequencymodulated carrier energy. There are various practical disadvantages inthe system disclosed in the aforesaid Roberts application. Ideally, therecording of frequency modulated signals should not require recordcutters of special design, nor should special types of record surfacesbe utilized. Further, it is desirable to have a system, such as thepresent, applicable to film, magnetic tape or any other recording systemcapable of 20 kilocycle (kc.) reproduction.

It can be stated, therefore, that it is one of the main objects of mypresent invention to provide a'system of recording sound waves by meansof frequency modulated signals which are produced by modulating localoscillations with the aforesaid sound waves. One of the outstandingadvantages of th present invention resides in the fact that the recordcutter may be of any well known type adapted for recording frequenciesfrom 10,000 to 20,000 cycles.

Another important object of this invention is to provide a method ofrecording sound waves wherein the latter are employed to frequencymodulate a carrier, at 15 kilocycles (kc.), up to a maximum deviation of2 to 3 kc. on either side of the mean carrier frequency,

Yet another important object of this invention is to provide a systemfor recording sound waves by virtue of a frequency modulation method,wherein the oscillator 'to be modulated infrequency is of theresistance-capacity type.

acteristic of my invention are set forth with particularity in theappended claims; the invention to the input grid 24, the cathode 25being con-' (Cl. 179L00.1)

itself, however, as to both its organization an method of operation willbest be understoodby reference .to the following description taken inconnection with the drawing in which I have indicated diagrammaticallyseveral circuit organizations whereby my invention may be carried intoeffect.

In the drawing:

Fig. 1 shows a circuit diagram of a recording system embodying theinvention,

Fig. 2 shows a reproducing system that may be used for'records producedby the system of Fig. 1.

Referring to Fig. 1, there will first be explained the oscillatorcircuit which is to be modulated in frequency by the sound waves whichare to be recorded. The oscillator is of the resistancecap'acity (R-C)type. It comprises a pentode I, which may be of the 1852 type (or 6A0?type) and a triode 2 which may be of the 6J5 type. The +B terminal ofthe direct current supply source is connected through voltage reducingresistor 3 to the plate 4 of tube I, and to the control grid 5 of tube2. The plate 6 of tube 2 is connected to the lower end of resistor 3,and the screen grid of tube i is also connected to this lower end. Thecathode l of tube i is connected to ground through a biasing resistor 8,and-thelatter is shunted by a condenser 9 of low impedance to theoscillator frequency currents. The cathode ID of tube 2 is connected toground through a resistor II which is an element of theresistancecapacity network of the oscillator circuit.

The resistance-capacity (R-C) network of the oscillator circuitcomprises the shunt resistors ll, l2, l3, l4 and IS. The condensers l6,II, It .and i9 arearranged in series relation between the cathode end ofthe resistor H and the lead 20. The lead 20 is'connected between theupper end of resistor l5 and the input, or control, grid 2| of tube l.The resistor I2 is connected between the junction of condensers l6 andI1 and ground, The resistor i3 is connected from the junction ofcondensers l1 and I8 to a lead 22. Resistor I4 is connected from thejunction of condensers i8 and ill to ground.

A twin triode tube 23, which may be of the 608G type, has one triodesection thereof arranged to act as the audio wave input section. Hence,the audio 'waves to be recorded are applied nected to ground through theunbypassed resistor 26. The plate 21 of triode section 25-24-21 isconnected in common to the cathode 28 of the other triode section 23.The lead 22 is connected quency of oscillations.

to the common connection of 21-28. Cathode 28 supplies electrons "to theplate 30, while control grid 3| is connected to an intermediate point.on the resistor 32 connected to ground from the plate 30. Positivevoltage can be applied to grid 3|, because the cathode of this sectionis also at positive potential with respect to ground by an amountdetermined by the bias existing on grid 24 and the position of the tapon resistor 32. The plate 30 may be connected to any desired source ofdirect current. By using the two triode sections in series it ispossible to provide in effect a tapped resistor, the bottom effectiveresistance of which depends upon the bias existing on grid 24 either asfixed bias or due to the incoming audio signal. Since the lower triodesection is in series with R13, whose normal value is about one half thatof the other resistors, we are able to effectively change Rn from alittle more than one half value to full value of the other resistors inthe network, depending upon the magnitude of the audio voltage appliedto grid 24.

The frequency modulated oscillations are derived from a path comprisingcondenser 40 and resistor 4| connected in series between the upper endof resistor II and ground. The adjustable contact 42 connects to thepower .amplifier. Contact 42 may be adjusted to any point along resistor4I thereby to regulate the magnitude of the frequency modulatedoscillation energy applied to the power amplifier which precedes therecord cutter. The resistor-capacity network connected between thecathode circuit of oscillator triode 2 and the input grid 2| of tube Ihas a natural, or mean, frequency Fc of approximately 15 kc. Theresistor I5 is made adjustable so as to set the initial, or mean,frequency of the oscillator;

The oscillator circuit itself provides sustained oscillations ofsubstantially constant amplitude 40 at its chosen frequency by virtue ofthe regenerative feedback from the cathode circuit of tube 2 to its gridcircuit through the resistor-capacity network and tube I. The oscillatoras such consists of both the tubes I and 2. The R-C delay networkcomprising condensers I6, I'I,' I8, I9 andresistors II, l2, I3, I4 andI5 is used as the cathode load circuit of the tube 2. 'I'he propagationconstant of the R-C network is such that for a given required frequencythe grid of tube I will be positive, when the cathode of 2 is negative.In other words a half cycle delay occurs between the cathode of 2 andthe grid of I. Since this is the case, and due to the amplifyingproperties of tube I, sustained oscillations can be produced thefrequency of which depends upon 5 the time delay characteristics of theR-C network. For a frequency value of about 15 kc. the resistor branchesare about 20,000 ohms, and the capacity values are of the order of 300micromicrofarads. It can thus be seen that by changing the effectivevalues of R13 and R15 it is possible to change the propagation constantof the R-C network, and consequently the fre- Tube I is used as an am- 5plifier, and tube 2 as a directly-coupled cathode follower amplifier,the combination producing oscillations by virtue of thetime delay R-Cnetwork and regenerative feedback to tube I.

Application of the audio waves to grid 24 acts 7 to vary the magnitudeof the frequency Fc. This variation is in accordance with the amplitudeof the sound waves. The source of audio Waves connected to grid 24 maybe a microphone, the detector output terminals of a radio receiver, a 75telephone line and the like. The mean frequency of the oscillator willbe varied, or deviated, in direct proportion to the amplitude of thesound waves, while the rate of deviation will be dependent upon theaudio frequencies per se. It is desirable to have a maximum deviation of2-3 kc. to either side of the value Fe. In this way the output of thepower amplifier may be fed to a record cutter of the type capable ofrecording frequencies from 10 to 20 kc. Such cutters are well known inthe art of recording sound waves.

One of the main advantages in recording the sound waves as frequencymodulated signals resides in the fact that background noise and scratchnoise is effectively eliminated during reproduction. The manner ofreproducing the record will be clear to those skilled in the art. Any ofthe reproducing systems shown in the aforesaid Roberts application maybe employed. In Fig. 2 there is schematically shown a sys tem which canbe employed. In this case the record is placed on the usual turntable,and there is employed a record pickup device which feeds the outputthereof to a potentiometer 50. The amplifier 5I amplifies the frequencymodulation signal energy, and the amplified signal energy is fed to atube 52 which has its cathode 53 connected to ground through theresistor 54.

The output circuit of amplifier 52 is a resonant circuit 55 tuned to themean frequency of 15 kc.', and it is shunted across the cathode resistor54. The main advantage of using tube 52 as a cathode follower lies inthe low resistance of resistor 54, which means that the primary circuit55 may be of low impedance thus requiring smaller components andresulting in 'a higher "62 circuit. The numerals 56 and 51 designate apair of resonant circuits which are respectively tuned to opposite sidesof the mean frequency value of 15 kc., and the opposite mistuning is byequal frequency values. The junction of the resonant circuits 56 and 51is connected to the midpoint of an output resistor 58, the lower end ofwhich is grounded and the equal sections of which are shunted by highfrequency bypass condensers. The diode rectifiers 59 and 60 areconnected in opposed relation as shown. ,It will be recognized by thoseskilled in the art that the discriminator-rectifier circuit is one whichis well known in the art. There will be developed across the resistor'58the audio wave voltage which was originally applied to the grid 24 oftube 23. Indeed, this type of discriminator-rectifier network is so wellknown that it need not be described in any further detail.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particularorganizationsshown and described, but that many modifications maybe madewithout departing from the scope of my invention, as set forth in theappended claims.

- What I claim is:

1. In a sound recording system of the type wherein the sound waves areemployed to frequency modulate superaudible oscillations, thecombination, which comprises a pair of electron discharge tubesarrangedin an inductance-free regenerative feedback circuit to produce said 0oscillations at a predetermined frequency and at constant amplitude, andmeans responsive to ap plied sound waves for deviating the frequency ofsaid oscillations to a maximum value of the order of 2 to 3 kilocycleson either side of said predetermined frequency.

'2. In a sound recording system of the type wherein the sound wavesareemployed to frequency modulate oscillations, the combination whichcomprises a pair of electron discharge devices arranged inresistance-capacity regenerative feedback circuit free of inductance toproduce constant amplitude oscillations at a predetermined frequency ofthe order of kilocycles, means including a resistor element of saidfeedback circuit, responsive to applied sound waves, for deviating thefrequency of said oscillations to a maximum value on either side of saidpredetermined frequency which is a relatively large percentage of thelatter value.

3. In a sound recording system of the type wherein audio frequency wavesare employed to frequency modulate superaudible oscillations, the

a combination which comprises a pairof electron discharge tubes arrangedin a regenerative feedback circuit to produce said superaudiblefrequency oscillations of a predetermined frequency, said feedbackcircuit being free of inductance and consisting solely of seriescapacity and shunt resistance chosen to provide said frequency, meansresponsive to applied sound waves for deviating the frequency of saidoscillations to a maximum value on,either side of said predeterminedfrequency which is a relatively large percentage of the latter value,and said last means comprising an electron discharge device in circuitwith a resistance element of said feedback circuit.

4. In a solund recording system of the type wherein sound waves areemployed to frequency modulate oscillations, the combination whichcomprises a pair of electron discharge tubes, a resistance-condensernetwork free of inductance providing a feedback circuit between thecathode of one tube and the input electrode of the other of 15kilocycles, and means responsive to applied sound waves for deviatingthe frequency of said oscillations to a maximum value of the order of3000 cycles on either side of'said predetermined frequency.

'5. In a sound recording system, a source of oscillations comprising apair of electron discharge tubes, means coupling the output elec- 'trodeof one of the tubes to an input electrode 6.'In a sound-recordingsystem, a source of oscillations comprising a pair of electron dischargetubes, means coupling the output electrode of one of the tubes to aninput electrode of the second tube, said second tube including a cathodeload consisting of a resistor-condenser network free of inductance, saidload havin an output connection to an input electrode of the first tubeand functionin as a regenerative feedback path, a third electrondischarge tube having its space current path in series with a resistorof said feedback path, a source of audio frequency wave energy connectedto an electrode of the third tube a point on said cathode load. a fourthelectron discharge device having its space current path in series withthe space current path of the third tube, and a source of direct currentfor energizingsaid third tube and said fourth device.

CHESTER M. SINNETT.

